Aqueous silica suspensions and their applications in compositions based on inorganic binder

ABSTRACT

This invention relates to an aqueous suspension including a mixture of at least one aqueous suspension of precipitated silica and of at least one latex, in which the said aqueous suspension of precipitated silica has a solids content of between 10 and 40% by weight, has a viscosity lower than 4×10 −2  Pa s at a shear of 50 s −1  and, after centrifuging at 7500 revolutions per minute for 30 minutes, produces a supernatant containing more than 50% of the weight of the silica initially in suspension.  
     This suspension can be employed in compositions based on an inorganic binder and in concrete compositions.

[0001] The present invention relates to aqueous suspensions includingprecipitated silica and to their use in the preparation of compositionsbased on cement or on similar inorganic binder or compositions derivedtherefrom.

[0002] The invention applies to all types of compositions including aninorganic binder such as cement, a slag or the like, as base ingredientor in combination with other constituents.

[0003] For the purpose of the present description “cement composition”will be intended to mean a composition based on cement or any othersimilar inorganic binder and water. These compositions constituteproducts which are used as they are, especially as coating, or elsewhich are intended to incorporate especially inorganic fillers ofvariable particle size.

[0004] Such compositions enclosing an inorganic binder and an inorganicfiller in granulate form are considered very generally as concretes.

[0005] Although the specialist customarily describes concrete morespecifically as a composition in which the inorganic filler is based onrelatively coarse granulates (of the order of 4 to 15 mm), and mortar asa composition in which the inorganic filler is based on less coarsegranulates (smaller than 4 mm), the term “concrete” will be employed inthe present description without any distinction to denote all kinds ofcompositions, whatever their particle size, in order to simplify thedescription.

[0006] It is desirable to have the ability to control the properties ofthese various compositions both during their manufacture and their use,as well as the products derived therefrom after setting.

[0007] Thus, concrete compositions for large structures (production ofwalls, veils, posts, cement finishes, slabs, industrial floors) mustsatisfy individual requirements with regard to:

[0008] rapid setting kinetics for early removal of shuttering;

[0009] low plastic shrinkage and absence of cracks particularly in thecase of cement finishes;

[0010] good mechanical properties at 28 days;

[0011] durability (resistance to abrasion, low permeability to gases andto liquids);

[0012] leakproofing in the particular case of the underground concretesor marine concretes in contact with water.

[0013] In addition, the concretes may either be manufactured on the worksite or manufactured in a central plant and transported to the site ofuse (concrete ready for use). In order that the composition may retainacceptable properties, even for a short period, its stability in thehydrated state must be ensured, especially by preserving its fluidityintact and by limiting the segregation of the materials in suspension.

[0014] More specifically, architectural concretes, that is to sayvisible concretes, must additionally satisfy requirements whereaesthetics are concerned:

[0015] homogeneous surface appearance;

[0016] reduction of efflorescences which form surface salt deposits (inparticular in the case of acid-treated concretes).

[0017] Prefabricated concretes for the production of facade components,paving stones, slabs and pipes must more particularly have a lowpermeability to gases and to liquids for an optimum durability, as wellas the same aesthetic properties as the architectural concretes for thevisible components.

[0018] The mortar or rendering compositions for the secondary work(finishing renderings, smoothing renderings, facade renderings, adhesivemortars, spray renderings) must themselves also satisfy a certain numberof individual requirements:

[0019] good water retention to avoid the loss of water by entry into theporous substrate or by preferential surface drying (risk of cracking andof surface powder formation);

[0020] good adhesiveness to the substrate;

[0021] resistance to abrasion and impact strength;

[0022] good fluidity combined with good water retention (especially inthe case of smoothing renderings), these contradictory properties beingdifficult to obtain together.

[0023] In general, during the utilization, critical parameters are thefluidity to ensure the processing, the degree of bleeding or theadhesiveness to the substrate and water retention.

[0024] To satisfy these requirements, appropriate additives are usuallyincorporated into cement compositions or in compositions derivedtherefrom. However, it is not rare for an additive to produce, besidesthe positive effect for which it is employed, a detrimental effect onanother property. Thus, a plasticizing agent may improve the fluidity ofa composition, but the high contents sometimes necessary to reach thedesired result very clearly promote bleeding and diminish the settingkinetics and hence the acquisition of mechanical strength at an earlyage (fight against cracking, suitability for removal of shuttering,early commissioning, etc.).

[0025] Similarly, setting retarders which allow a composition to beconserved and transported for a certain time after its preparation alsohave a very marked effect on bleeding and on the setting kinetics.

[0026] Such difficulties appear in particular in the case ofcement-based compositions and of self-levelling fluid concretes which,according to conventional specifications, must exhibit strictrheological characteristics in respect of settling (measured with theAbrams cone) or static or dynamic spreading.

[0027] The solutions which make it possible to obtain fluid concretes ormortars, and which are known so far, consist either in optimizing theparticle size curve by the introduction of fines or of ultrafines asdescribed in patent EP-A-0 184 386, this solution exhibiting, besidesthe awkwardness of making it necessary to handle large quantities ofpowder, the disadvantage of being highly sensitive to small variationsin the content or quality of the products employed, which in practicemakes it difficult to employ on a work site, or to add large quantitiesof plasticizers, retarders and optionally water-retaining agents tomaintain the handleability of the material for a sufficient period,which generally entails a delay in setting which impairs the efficiencyof the work site (delayed removal of shuttering or need for finishing inthe case of early shuttering removal) and a great sensitivity to smallchanges around the optimized composition.

[0028] In practice, one -is often confronted with problems ofprohibitive decrease in the handleability in the event of meteringerror, of segregation of the mortar or of the concrete, of highbleeding, of lengthening of the setting time and of lowering of thefinal mechanical properties, in particular in the event of overdosingwith water.

[0029] In fact, it is found very difficult to produce cementcompositions or compositions derived therefrom, which meet all theessential requirements that are desired with a view to an application.

[0030] From U.S. Pat. No. 5,149,370 and GB-2 212 489 it was known toimprove the bleeding and segregation properties of compositions byadding silica sols thereto. However, the properties of thesecompositions were still inadequate.

[0031] The present invention proposes to meet this demand for improvedadditives which make it possible to combine a number of advantageousproperties from among those referred to above.

[0032] The aim of the invention is to provide a new additive for cementcompositions or compositions derived therefrom, making it possible todecrease the bleeding and segregation effects of the concretecompositions.

[0033] A further object of the invention is to provide an additivemaking it possible to combine an improved fluidity, good waterretention, even in the case of high contents of water and/or offluidizing or plasticizing agents, and to impart a good durability tothe set product, especially a reduction in the permeability to water andto gases, making it possible to reduce the efflorescences.

[0034] To this end, a further object of the invention is an aqueoussuspension including a mixture of at least one aqueous suspension ofprecipitated silica and of at least one latex, in which the said aqueoussuspension of precipitated silica has a solids content of between 10 and40% by weight, has a viscosity lower than 4×10⁻² Pa s at a shear of 50s⁻¹, and, after centrifuging at 7500 revolutions per minute for 30minutes, produces a supernatant containing more than 50% of the weightof the silica initially in suspension.

[0035] The first essential component of the mixture forming the aqueoussuspension which is the subject-matter of the invention is an aqueoussuspension of precipitated silica of high solids content, exhibiting alow viscosity and good stability with time.

[0036] Such suspensions are described, together with the process fortheir manufacture, in FR-A-2 722 185.

[0037] The solids content of the said suspension is preferably between15 and 35% by weight. The viscosity of the said suspension isadvantageously lower than 4×10⁻² Pa s at a shear of 50 s⁻¹.

[0038] These suspensions are very stable and their stability can beassessed by virtue of a sedimentation test which consists incentrifuging the said suspension at 7500 rev/min for 30 minutes. Thequantity of silica present in the supernatant obtained at the end ofthis centrifuging, measured after drying the supernatant at 160° C.until a constant weight of material is obtained, represents more than50%, preferably more than 60% of the weight of the silica present in thesuspension.

[0039] The quantity of silica present in the supernatant obtained aftercentrifuging advantageously represents more than 70%, in particular morethan 90% of the weight of the silica in suspension.

[0040] Another essential feature of these suspensions concerns theparticle size of the silica particles in suspension.

[0041] In fact, besides their high viscosity, the concentrated silicasuspensions known so far exhibit the disadvantage of comprisinglarge-sized agglomerates in suspension, which give rise to sedimentationin the course of time.

[0042] The particle size distribution of the materials in suspension canbe defined by means of the median diameter D₅₀, which is the particlediameter such that 50% of the population of particles in suspension havea smaller diameter.

[0043] Similarly, D₉₅ denotes the particle diameter such that 95% of thepopulation of particles in suspension have a smaller diameter.

[0044] Another characteristic value of the suspensions is thedeagglomeration factor F_(D). This factor, which is proportionallyhigher the more the silica suspension is deagglomerated, is indicativeof the proportion of fines, that is to say of the proportion ofparticles smaller than 0.1 μm in size, which are not detected by acommonplace particle size analyser.

[0045] The particle size characteristics of the silica suspensions aredetermined by virtue of a particle size measurement performed on thesuspensions with the aid of a Sympatec particle size analyser.

[0046] F_(D) is measured by introducing into a particle size analyser aknown volume V of suspension diluted so as to obtain a silica content of4% by weight, and is equal to the ratio (10×V in ml/opticalconcentration detected by the particle size analyser).

[0047] The silica agglomerates present in these suspensions are of smallsize.

[0048] The particle size distribution of the agglomerates in suspensionis preferably such that their median diameter D₅₀ is smaller than 5 μmand the deagglomeration factor FD is higher than 3 ml.

[0049] Advantageously, the diameter D₅₀ is smaller than 2 μm, the factorF_(D) is greater than 13 ml and, in addition, the diameter D₉₅ issmaller than 20 μm.

[0050] The second essential component of the mixture forming the aqueoussuspension which is the subject-matter of the invention is a latex,namely an aqueous suspension of particles of natural or synthetic resin.

[0051] The said particles are advantageously products of polymerizationof at least one monomer containing ethylenic unsaturation.

[0052] The latex itself is preferably produced by emulsionpolymerization of at least one monomer containing ethylenicunsaturation.

[0053] In particular, the monomer containing ethylenic unsaturation maybe advantageously selected from styrene, butadiene, acrylic acid,methacrylic acid, esters, preferably C₁-C₁₂, of acrylic or methacrylicacid, vinyl esters and mixtures thereof.

[0054] The latex which can be employed in accordance with the inventionmay also include particles of homopolymer or copolymer resin, an examplebeing a styrene-butadiene rubber latex.

[0055] The particle size of the latex can vary in accordance with theintended application. In a first advantageous embodiment this size isfrom 0.1 to 5 μm. Latices in which the particle size is from 0.1 to 0.3μm may be mentioned in particular, preferably styrene/utadiene latices,or else latices in which the particle size is from 1 to 5 μm, preferablyacetate/versatate latices. In a second advantageous embodiment this sizeis at most 100 nm; these nanolatices may be of very diverse nature.

[0056] The abovementioned two components are used in combination inproportions which are appropriate for the desired application.

[0057] In general it is preferable that the aqueous suspension accordingto -the invention should include from 3 to 25 parts by weight of silica,expressed as dry weight, per 100 parts by weight of suspension, moreadvantageously from 5 to 20 parts by weight of silica per 100 parts ofsuspension.

[0058] It is preferable, furthermore, that the dry weight of the latexshould represent the value of 5 to 50 parts per 100 parts by weight ofaqueous suspension according to the invention, advantageously from 10 to40 parts per 100 parts of suspension.

[0059] The aqueous suspensions according to the invention may beincorporated into cement pastes or more generally cement compositionsbased on inorganic binder and water, which they provide especially withresistance to segregation and water retention, in combination with anincrease in fluidity as a result of a synergistic effect between thesilica suspension and the latex.

[0060] The invention therefore also provides compositions based on aninorganic binder and water, which are characterized in that they includean aqueous suspension as defined above.

[0061] The inorganic binder may be of any known type, especially cementof Portland CPA H.P, CPA 55, CPJ 45, CPA CEM I, CPA CEM I PM, CPA CEM IPMES, CPJ CEM II, CPJ CEM II PM, CPJ CEM II PMES, CHF CEM II and CLK CEMII type, blast furnace slag or pozzolanic binders. The ratio of thewater to the binder is variable and depends above all on the desiredfluidity of the composition. This ratio may be in particular from 0.3 to2, preferably from 0.3 to 1.5.

[0062] In these compositions it is preferable that the silica shouldrepresent from 0.3 to 5% by weight relative to the binder, expressed inrelation to dry weight, preferably from 0.8 to 1.5%.

[0063] It is also advantageous that the dry weight of the latex shouldrepresent the value of 0.2 to 50% by weight relative to the binder,expressed in relation to dry weight, preferably from 3 to 30%.

[0064] To supplement the effect provided by the aqueous suspensionaccording to the invention, the compositions may additionally include atleast one plasticizing agent. This agent may be selected from thesubstances commonly employed for this purpose in cement compositions. Itwill advantageously be selected from lignosulphonates, casein,polynaphthalene, in particular alkali metal polynaphthalene sulphonates,melamines, polymelamines, formaldehyde derivatives, alkali metalpolyacrylates, alkali metal polycarboxylates and grafted polyethyleneoxides.

[0065] Such an agent may advantageously be employed in a proportion of0.1 to 10% by weight relative to the binder.

[0066] This agent may be introduced separately from the aqueoussuspension according to the invention or else simultaneously by means ofa suspension including the silica suspension, the latex and the saidagent at the same time.

[0067] Similarly, the compositions may additionally include at least onewater-retaining agent which may be selected from substances commonlyemployed for this purpose in cement-based compositions. It willadvantageously be selected from optionally modified polyvinyl alcohols,polyethylene glycols, polyoxyethylenes, acrylic polymers, especiallypolyacrylamides, polysaccharides of bacterial origin, like xanthan gum,guar gums, cationized guar gums, carob seed extracts, alginates,pectins, celluloses, cellulose ethers, especially carboxyalkylcelluloses, alkyl celluloses, hydroxyalkyl celluloses such asmethylhydroxypropyl celluloses, polyvinylpyrrolidone, sugars, especiallydextrose, ribose, corn starches, wheats, cationized or otherwise,lignites, leonhardites and derived products, alkali metal polyacrylatesand polystyrenesulphonates.

[0068] Such an agent may advantageously be employed in a proportion of0.01 to 10% by weight relative to the binder.

[0069] Furthermore, the compositions may also advantageously include asetting accelerator such as aluminium sulphate, in a content of 0.01 to3% by weight relative to the binder (expressed in relation to dryweight). In the case of aluminium sulphate this content is expressed asweight of anhydrous aluminium sulphate.

[0070] This agent may be introduced separately from the aqueoussuspension according to the invention or else simultaneously by means ofa suspension including the silica suspension, the latex and the saidagent at the same time.

[0071] Reinforcing fibres enabling the resistance to cracking to beimproved will also advantageously be employed in the compositions. Thesefibres are preferably selected from polyvinyl alcohol, polypropylene,polyethylene, steel, polyacrylonitrile, cellulose, carbon, kevlar,polyamide and polyester fibres.

[0072] The cement compositions based on an inorganic binder and waterwhich have just been described can be employed as such or combined withother materials, especially to form concrete (or mortar) compositions.As has been explained above, concrete is intended to mean the mixture ofan inorganic binder, water and granulates of variable particle size,especially sand-gravel mixes, sands and optionally fines, or evenultrafines.

[0073] In this regard a further object of the invention is concretecompositions including an aqueous suspension described above. Thesecompositions can be obtained by mixing the suspensions according to theinvention with an inorganic binder, granulates and optionally additionalwater, or else by mixing a cement composition described above withgranulates and optionally additional water.

[0074] Consequently another object of the invention is the use of theaqueous suspensions or of the cement compositions described above inconcrete compositions, in particular speciality concretes of theabovementioned types and secondary work coats.

[0075] Aqueous suspensions or cement compositions will advantageously beemployed which correspond to at least one of the preferredcharacteristics indicated above with regard to the relative contents ofsilica and latex, and optionally of plasticizing or water-retainingagents and, where appropriate, of cement.

[0076] The proportion of granulates in these concrete compositions maybe chosen in a manner known per se within the usual ranges correspondingto the desired type of concrete.

[0077] The invention finds a particular application in so-calledspeciality concretes which must exhibit particular properties withregard to surface appearance, mechanical strength, durability of thefinished product, fluidity in processing and low tendency tosegregation.

[0078] It is possible, for example, to mention fluid concretes,concretes for the manufacture of slabs (cement finishes and fluidmortars for finishes), submarine concretes which must be easily pumpableand injectable, facing concretes, concretes prefabricated bycentrifuging or hot pressing, surface smoothing and finishing coats,especially for floors, coloured concretes which must have a homogeneoussurface without efflorescence and a relatively light base colour, aswell as lightweight concretes, concretes for industrial floors,leakproof concretes and cementing concretes for oil wells, which musthave a low porosity, the properties of all these concretes being seen tobe improved by virtue of the invention.

[0079] More particularly, in the case of fluid concretes and cements,the high fluidity which is frequently desirable is obtained by virtue ofthe invention while the ability of the compositions to retain water isimproved, and this makes it possible to facilitate the positioning andto reduce the problems of cracking due to a loss of water entailing aplastic shrinkage shortly after the positioning.

[0080] A further object of the invention is thus the use of the aqueoussuspensions described above as water-retaining agent in cementcompositions or concretes. This use, which makes it possible to reduceplastic shrinkage and, as a result, cracking, is particularlyadvantageous for cement finishes and smoothing coats.

[0081] The advantageous properties imparted to the cement-basedcompositions and to the concrete compositions by the aqueous suspensionsaccording to the invention include more particularly the low tendencyfor the segregation of the hydrated compositions, especiallycompositions with a high water content.

[0082] In this regard, a further object of the invention is the use ofaqueous suspensions as defined above, as antisegregation agent in cementcompositions based on an inorganic binder and water and in concretecompositions, especially in compositions where the water/cement, or moregenerally water/binder, weight ratio is from 0.3 to 2, preferably from0.3 to 1.5.

[0083] The invention is also very advantageous for the production ofprefabricated facing concretes or leakproof concretes, because it makesit possible to obtain better dispersion of the cement and betterhomogeneity. The surface appearance is thus improved in terms ofuniformity and durability of the set product by virtue of the limitedentry of water and of gases.

[0084] In the case of the facing products and of smoothing and finishingcoats, this is accompanied by a lightening of the colour and animprovement in surface appearance, by better resistance to carbonateformation and a limitation in the appearance of efflorescences, as wellas a reduction in the corrosion of the reinforcements.

[0085] Another subject-matter of the invention is the use of aqueoussuspensions described above as agent for reducing the permeability togases and to liquids in the compositions based on an inorganic binderand water and in concrete compositions.

[0086] The resistance to the entry of water is a decisive advantage forthe leaktightness of structures situated in a region liable to floodingor wet or of structures intended to receive water (reservoirs, dams,aqueducts, swimming pools and water conduits).

[0087] Other advantages of the invention will appear more clearly in thelight of the examples which follow.

EXAMPLE OF PREPARATION Preparation of a Suspension of PrecipitatedSilica

[0088] 1. A cake C1 of precipitated silica is prepared as follows.

[0089] Into a stainless steel reactor fitted with a stirring systemusing propellers and with heating using a jacket are introduced:

[0090] 346 liters of water,

[0091] 7.5 kg of Na₂SO₄ (electrolyte),

[0092] 587 liters of aqueous sodium silicate which has an SiO₂/Na₂Oweight ratio equal to 3.50 and a relative density at 20° C. equal to1.133.

[0093] The silicate concentration (expressed as SiO₂) in the base stockis then 85 g/l. The mixture is heated to 79° C. while being keptstirred. 386 liters of dilute sulphuric acid with a relative density at20° C. of 1.050 are then introduced into the mixture until a pH value of8 (measured at the temperature of the mixture) is obtained. Thetemperature of the reaction mixture is 79° C. during the initial 25minutes and is then raised from 79° C. to 86° C. over 15 min, and thenmaintained at 86° C. until the end of the reaction.

[0094] Once the pH value of 8 has been reached, 82 liters of aqueoussodium silicate with an SiO₂/Na₂O weight ratio of 3.50 and a relativedensity at 20° C. of 1.133 and 131 liters of acid of the type describedabove are introduced jointly into the reaction mixture, thissimultaneous introduction of acid and of silicate being carried out insuch a way that the pH of the reaction mixture is constantly equal to8±0.1. After all the silicate has been introduced, the dilute acidcontinues to be introduced for 9 min so as to bring the pH of thereaction mixture to a value of 5.2. The introduction of acid is thenstopped and the reaction slurry is kept stirred for an additional 5 min.

[0095] The total duration of the reaction is 118 min.

[0096] A slurry of precipitated silica is obtained which is filtered andwashed by means of a filter press in such a way that a silica cake G1 isfinally recovered, whose loss on ignition is 78% (hence a solids contentof 22% by weight) and whose Na₂SO₄ content is 1% by weight.

[0097] 2. 4 kg of the cake G1 (obtained by press filtration andexhibiting a solids content of 22% by weight and a Na₂SO₄ content of 1%by weight), preheated to 60° C., are introduced into a Cellier crumblermixer.

[0098] Then, during the deflocculation of the cake, 13.1 ml of asolution of sodium aluminate (which has an Al₂O₃ content of 22% byweight and a Na₂O content of 18% by weight; relative density: 1.505) and7.47 ml of a solution of sulphuric acid containing 80 g/l (relativedensity: 1.505) are introduced simultaneously so as to maintain the pHat a value of 6.5.

[0099] The Al/SiO₂ weight ratio is approximately 2600 ppm.

[0100] Maturing is allowed to take place for 20 minutes while themechanical deflocculation is continued.

[0101] The silica suspension G2 obtained is characterized by:

[0102] a viscosity of 0.06 Pa s (measured under a shear of 50 s⁻¹ for 1minute);

[0103] a particle size such that D₁₀=5 μm, D₅₀=19 μm, D₉₀=60 μm.

[0104] After one week's storage, the following are observed:

[0105] the formation, at the bottom of the storage container, of asediment which is excessively difficult, or even impossible, toredisperse;

[0106] an increase in the viscosity of the suspension: its viscosity isthen 0.45 Pa s (measured under a shear of 50 s⁻¹ for 1 minute).

[0107] 3. The chamber of a Netzch LMEI mill is fed with 2 liters ofsuspension G2 (taken after the crumbling), exhibiting a solids contentof 22% by weight, at a flow rate of 5 liters/h; the filling ratio of thechamber with alumina beads (diameter: 0.6-1 mm) is 75% and the speed ofrotation of the shaft is 2000 rev/min.

[0108] At the end of this wet milling stage a suspension is obtained,characterized by:

[0109] a viscosity of 29 mPa s (measured under a shear of 50 s⁻¹ for 1minute);

[0110] a particle size such that D₁₀=1.13 μm, D₅₀=4.1 μm, D₉₀=9.33 μm;

[0111] a solids content of 22% by weight;

[0112] a quantity of silica found in the supernatant of 63% (accordingto the test for sedimentation under centrifuging, defined above).

[0113] 4540 g of the silica suspension thus prepared are introduced intoa stirred reactor and the pH is adjusted to 9.5, with stirring, with theaid of concentrated sodium hydroxide solution, to stabilize the mixture.

[0114] 5000 g of a styrene-butadiene rubber latex with a solids contentof 50%, exhibiting a particle size of 0.1-0.2 μm, marketed byRhône-Poulenc under the name SB 112, are then introduced with stirringat a flow rate of 100 g/min.

[0115] After half an hour's stirring the pH is readjusted to 9.5 and asuspension exhibiting a viscosity of 30 mPa s under a shear of 50 s-1 isobtained.

[0116] This suspension is perfectly stable with regard to sedimentationfor more than three months.

[0117] It contains approximately 10% by weight (dry) of silica and 26%by weight (dry) of latex.

EXAMPLE OF APPLICATION 1 Production of a Facing Concrete

[0118] Test 1

[0119] A concrete composition is prepared according to the followingformula: Granulates (5-15) 1000 kg Granulates 0-4 (sand) 800 kg HPRcement 425 kg Plasticizer (Rheobuild 2000 PF marketed 1.7 kg by MBT)(0.1 % relative to the weight of cement) 147 l Water

[0120] The granulates are introduced into a Hobart type planetary mixerand are mixed for one minute, then the cement is introduced and mixingis continued for 30 seconds.

[0121] Next, while the mixing is continued, the water and theplasticizer are introduced, together with 42.5 kg (10% relative to thecement) of the aqueous suspension from the reference example.

[0122] The quantity of silica added corresponds to 1% of the weight ofcement introduced.

[0123] The water/cement weight ratio of this concrete composition istherefore 0.41.

[0124] Test 2

[0125] A concrete composition is prepared in a similar manner, theinitial water content being increased so as to obtain a water/cementweight ratio of 0.49, the quantity of all the other constituents beingkept identical.

[0126] Comparative Tests 1 and 2

[0127] Compositions similar to those of tests 1 and 2 are prepared, inwhich the introduction of the aqueous suspension from Example A isomitted but the water/cement ratio is preserved.

[0128] The characteristics of these four compositions are evaluated bycasting a concrete into identical leakproof moulds and by demouldingafter three days.

[0129] It is noted first of all that in tests 1 and 2 the fluidity ofthe concrete is better and the positioning in the shuttering takes placebetter. Furthermore, in tests 1 and 2 no segregation of granulates orbleeding is observed, and this was hitherto incompatible with goodfluidity.

[0130] The characteristics of the set concretes are also evaluated. Thewhiteness of the product is measured by means of a Xenotest calorimeterand quantified by means of the chromatic coordinates L*, a* and b* inthe CIE 1976 (L*, a*, b*) system as defined by the InternationalCommission on Illumination and listed in the Collection of FrenchStandards (AFNOR) calorimetric colour No. X08-12 (1983).

[0131] Better homogeneity of the surface and an appreciable lighteningin colour are noted in tests 1 and 2.

[0132] Possible formation of efflorescence is verified after 28 days. Atthis time no efflorescence has appeared in tests 1 and 2, whereas theproducts of the comparative tests exhibit numerous whitishefflorescences at the surface.

[0133] The results are recorded in Table 1 which follows. TABLE 2 Silicasuspension (% relative Whiteness Efflorescence Test W/C to cement) L a bat 28 days 1 0.41 10 58 −0.08 3.1 no 2 0.49 10 58.05 −0.07 3.25 no comp.1 0.41 0 55 −0.00 3.8 yes comp. 2 0.49 0 55 −0.03 3.99 yes

[0134] It would appear that the combined action of the silica and of thelatex contributes a filling of the porosity of the concrete, whichsooner or later reduces the efflorescences.

[0135] It is also observed that the water and gas permeability of theconcretes of tests 1 and 2 is reduced in comparison with those of thecomparative tests 1 and 2, which results in greater durability due toresistance to carbonate formation, to attack by corrosive water, etc.

[0136] In addition, the adhesiveness of the concrete to a substrate isimproved as a result of the presence of the silica suspension and of thelatex, as is the resistance to abrasion (decrease in dust formation).

[0137] A decrease in the number of cracks, due to an improvement inwater retention, is also observed.

EXAMPLE OF APPLICATION 2 Production of a Cement Composition of HighWater Content (or Grout)

[0138] This example aims to demonstrate the synergy effect produced bythe silica suspension and the latex on the rheological and stabilityproperties of cement pastes with a high water content.

[0139] A first series of tests not in accordance with the invention iscarried out, showing that the silica or the latex alone do not solve theproblems presented by these grouts, which are compared with two testsaccording to the invention.

[0140] Comparative Test 1

[0141] A grout is prepared including a silica suspension prepared in thesame way as in the example of preparation A but without adding latex. Itcorresponds to the following formula: - HTS cement 100 g - plasticizer(Melmentplast N40) 2.5 g - setting retarder (Melretard) 0.8 g - aqueoussilica suspension with a solids 5 g content of 23 % - water 24 g *marketed by CIA

[0142] The water/cement ratio is therefore 0.28.

[0143] The water, the plasticizer and the retarder are introduced into abeaker stirred with a paddle and the cement is then added with stirringfor 3 minutes. The silica suspension is then introduced with stirring,which is continued for 10 minutes.

[0144] Comparative Test 2

[0145] A similar grout in which the water/cement ratio is 0.35 isprepared in the same way.

[0146] Comparative Test 3

[0147] A grout is prepared corresponding to the following formula: - HTScement 100 g - plasticizer (Melmentplast) 2.5 g - retarder (Melretard)0.8 g - styrene-butadiene latex as 50 % aqueous 5 g suspension (SB112) - water 25 g

[0148] The water/cement ratio is therefore 0.28. The preparation issimilar to the comparative test 1, the latex being introduced insteadand in place of the silica suspension.

[0149] Test 1

[0150] According to the invention, a grout corresponding to thefollowing formula is prepared: - HTS cement 100 g - plasticizer(Melmentplast) 2.5 g - retarder (Melretard) 0.8 g - aqueous suspensionfrom example A 9.6 g that is: silica (dry weight) 1 g latex (dry weight)2.5 g - water 22 g

[0151] The water/cement ratio is 0.28. The preparation is similar tothat of the comparative examples.

[0152] Test 2

[0153] A grout similar to that of test 1 is prepared, in which the watercontent is adjusted to obtain a water/cement ratio of 0.35.

[0154] These five compositions are subjected to the followingTheological measurements performed on a Rheomat 115 apparatus equippedwith an MS DIN 145 cell. The grout is subjected to a shear increasingfrom 0 to 1000 s⁻¹ over 1 minute and then to a constant shear of 1000s⁻¹ for 1 minute, and the shear is decreased to 0 s⁻¹ over 1 minutewhile the viscosity is measured. The final viscosity is recorded.

[0155] The tendency to segregation and to bleeding is evaluated:

[0156] by a static method: the appearance of a test tube filled with 200ml of grout is observed over a period of 24 hours;

[0157] by a dynamic method under pressure: a given quantity of grout isplaced in an API filter, above which an air pressure of 7×10⁵ Pa isapplied; the quantity of water expelled from the paste is measured and,after drying at 160° C., the solids content of the upper part (air side)and of the lower part (filter side) of the filter cake obtained ismeasured.

[0158] The solids content of the lower or upper sample of the cake isdefined by the formula:${{solids}\quad {content}} = {\frac{\begin{matrix}{{{total}\quad {mass}\quad {of}\quad {the}\quad {sample}} -} \\{{mass}\quad {of}\quad {water}\quad {from}\quad {the}\quad {sample}}\end{matrix}}{{total}\quad {mass}\quad {of}\quad {the}\quad {sample}} \times 100}$

[0159] A difference between the two values indicates a heterogeneouscake, a lower solids content higher than the upper solids contentsignifying a segregation.

[0160] The results are expressed by the following values:

[0161] the percentage of water collected by filtration${Wc} = {\frac{{quantity}\quad {of}\quad {water}\quad {collected}}{{initial}\quad {quantity}\quad {of}\quad {grout}} \times 100}$

[0162] the water content of the cake after filtration${Cf} = {\begin{matrix}{{{initial}\quad {water}} -} \\\frac{{water}\quad {collected}{\quad \quad}{by}\quad {filtration}}{{mass}\quad {of}\quad {the}\quad {filtered}\quad {cake}}\end{matrix} \times 100}$

[0163] relative to the initial water content of the grout Ci with${Ci} = {\frac{{initial}\quad {water}}{{mass}\quad {of}\quad {the}\quad {initial}\quad {cake}} \times 100}$

[0164] It is observed that, in relation to the comparative test 2, thecombined addition of a silica suspension as defined in the invention andof a latex in test 2 results in bleeding properties which are superiorto those obtained with the silica suspension alone.

[0165] Furthermore, it is observed that the addition of latex alone inthe comparative test 3 results in a cement composition which has badbleeding and segregation properties.

[0166] Consequently, the -excellent antibleeding and segregationproperties of the additives according to the invention are whollyunexpected, insofar as a person skilled in the art would have expected adecrease in the bleeding and segregation properties on combining anaqueous silica suspension with a latex, and not an improvement.

[0167] The results are collated in Table 2 below. TABLE 1 Dynamicbleeding (retention of water under pressure) Viscosity Static bleedingUpper solids Lower solids Test Additive W/C mPa s Bleeding Packing Wc %Cf % Ci % content % content % Comp. 1 Silica 0.28 140  none none 7.115.7 22 83 84 alone (thixotropic behaviour) Comp. 2 Silica 0.35 65 lightyes 10.5 17.2 26 80.7 85.4 alone Comp. 3 Latex 0.28 80 3.5 ml high 5.517.3 22 0 (*) 87 alone 1 Silica + 0.28 93 none none 9.5 13.7 22 86 86.4latex 2 Silica + 0.35 50 none none 12.5 15.3 26 85 85 latex

[0168] From the above results it appears that the silica employed alonein the comparative test 1 produces a homogeneous grout which is stablebut which has a tendency to form a gel on standing (which is, however,destructured under low shear). Nevertheless, when the W/C ratioincreases, a tendency to bleeding and to packing (segregation) appears.In the dynamic bleeding test the cake obtained is more heterogeneous,which explains the packing observed in the static test. The silica is atthe limit of effectiveness.

[0169] In the comparative test 3, the tendency to bleeding and tosegregation is very great in static conditions and under pressure. Thelatex has no advantageous effect on these phenomena.

[0170] In test 1, a lesser tendency to gelling is noted in comparisonwith the comparative test 1, the behaviour of the grout beingquasi-Newtonian. The grout does not present any bleeding or segregationproblem: it remains perfectly homogeneous in both static and dynamicconditions, although it is much more fluid than the grout in thecomparative test 1.

[0171] In test 2, while the W/C ratio is 0.35, the grout remainsperfectly stable and homogeneous in static and dynamic conditions, incontrast to the grout of the comparative test 2.

[0172] Furthermore, although the grout is more fluid (50 mPa s against65) a much better water retention is observed in test 2.

1. Aqueous suspension including a mixture of at least one aqueoussuspension of precipitated silica and of at least one latex, in whichthe said aqueous suspension of precipitated silica has a solids contentof between 10 and 40% by weight, has a viscosity lower than 4×10⁻² Pa sat a shear of 50 s⁻¹ and, after centrifuging at 7500 revolutions perminute for 30 minutes, produces a supernatant containing more than 50%of the weight of the silica initially in suspension.
 2. Suspensionaccording to claim 1, wherein the latex includes a product ofpolymerization of at least one monomer containing ethylenicunsaturation, selected especially from styrene, butadiene, acrylic acid,methacrylic acid, esters of acrylic or methacrylic acid, vinyl estersand mixtures thereof.
 3. Suspension according to either of the precedingclaims, wherein the latex has a particle size of 0.1 to 5 μm. 4.Suspension according to either of claims 1 and 2, wherein the latex hasa particle size of at most 100 nm.
 5. Suspension according to any one ofclaims 1 to 4, including from 3 to 25 parts by weight of silica,expressed as dry weight, per 100 parts of suspension.
 6. Suspensionaccording to any one of claims 1 to 5, including from 5 to 50 parts byweight of latex, expressed as dry weight, per 100 parts of suspension.7. Composition based on an inorganic binder, especially cement andwater, including a suspension according to one of claims 1 to
 6. 8.Composition according to claim 7, including from 0.3 to 5% by weight ofsilica, expressed as dry weight, relative to the binder, preferably from0.8 to 1.5%.
 9. Composition according to either of claims 7 and 8,including from 0.2 to 50% by weight of latex, expressed as dry weight,relative to the binder, preferably between 3 and 30%.
 10. Compositionaccording to any one of claims 7 to 9, including at least oneplasticizing agent in a content of 0.1 to 10% by weight relative to thebinder.
 11. Composition according to claim 10, wherein the plasticizingagent is selected from lignosulphonates, casein, polynaphthalene, inparticular alkali metal polynaphthalene sulphonates, melamines,polymelamines, formaldehyde derivatives, alkali metal polyacrylates,alkali metal polycarboxylates and grafted polyethylene oxides. 12.Composition according to any one of claims 7 to 9, including at leastone water-retaining agent in a content of 0.01 to 10% by weight relativeto the binder.
 13. Composition according to claim 12, wherein thewater-retaining agent is selected from optionally modified polyvinylalcohols, polyethylene glycols, polyoxyethylenes, acrylic polymers,especially polyacrylamides, polysaccharides of bacterial origin, guargums, cationized guar gums, carob seed extracts, alginates, pectins,celluloses, cellulose ethers, especially carboxyalkyl celluloses, alkylcelluloses, hydroxyalkyl celluloses, methylhydroxypropyl celluloses,polyvinylpyrrolidone, sugars, corn starches, wheats, cationized orotherwise, lignites, leonhardites and derived products, alkali metalpolyacrylates and polystyrenesulphonates.
 14. Composition according toany one of claims 7 to 13, including a setting-accelerating agent in acontent of 0.01 to 3% by weight relative to the binder.
 15. Compositionaccording to any one of claims 7 to 14, including reinforcing fibres.16. Composition according to any one of claims 7 to 15, wherein theweight ratio of the water to the binder is from 0.3 to
 2. 17. Concretecomposition including an inorganic binder, granulates and an aqueoussuspension according to one of claims 1 to
 6. 18. Composition accordingto claim 15, wherein the weight ratio of the water to the binder is from0.3 to
 2. 19. Use of a composition according to any one of claims 7 to16 for the preparation of speciality concretes, especially fluidconcretes, concretes for the manufacture of slabs, submarine concretes,facing concretes, floor smoothing and finishing coats, leakproofconcretes, lightweight concretes or coloured concretes, concretes forindustrial floors, concretes prefabricated by centrifuging or hotpressing and cementing concretes for oil wells.
 20. Use of a suspensionaccording to any one of claims 1 to 6 as antisegregation agent in cementcompositions or in concrete compositions.
 21. Use of a suspensionaccording to any one of claims 1 to 6 as agent for reducing thepermeability to gases and to liquids in cement compositions or inconcrete compositions.
 22. Use of a suspension according to any one ofclaims 1 to 6 as agent allowing water retention in cement compositionsor in concrete compositions.